1. Field of the Invention
The invention relates to silicate materials comprising silicate domains and one or more substantially nonsilicate domains, in particular to porous silicate materials, a method for fabricating a silicate material, an optical article, a method for fabricating an optical article and a holographic medium.
2. Discussion of the Related Art
Silicate materials with designed porosity have many applications, some of which include applications in the chemical, optical and electronics industries. Porosity is typically introduced by incorporation of surfactant or polymeric templates into sol-gel-like reaction mixtures. The high surface area and specific pore structure of porous silicate materials contribute to their value in chemical separations, sensing, biomaterials, and catalysis. The optical quality of silicates in general suggests their use in fabricating claddings, waveguides, switches, holographic storage media, and other active elements, especially when filled with organic phases. The tunability of the electrical properties of the pores, which in principle can encompass air, solid insulators, semiconductors, metals, or other constituents, makes porous silicates attractive for electronic devices such as low-dielectric constant barriers or high-dielectric constant capacitors. Additionally, porous silicates, when impregnated with photosensitive materials, can serve as media for optical data recording.
It is known to prepare mesoporous silica plates with centimeter-scale lengths and widths and 0.5-mm thickness by codissolving Tetraethoxysilane (TEOS), a resin precursor, with surfactants and excess water in ethanol-heptane, then partially curing and casting the material. Ryong Ryoo et al., Optically Transparent, Single-Crystal-Like Oriented Mesoporous, Silica Films and Plates, J. Phys. Chem. B, Vol. 101, No. 50 (1997). In Ryoo""s process some solvent (but probably not all) is removed from the gelling mixture before casting the final shape. No resin is isolated in the process.
It is also known to employ TEOS as a resin precursor with block polyether glycol copolymer templates to form porous fibers and powders. Galen D. Stucky et al., Triblock-Copolymer-Directed Syntheses of Large-Pore Mesoporous Silica Fibers, Chem. Mater., Vol. 10, No. 8 (1998).
Thin films have been prepared by using vinyltrimethoxysilane as an additive in a tetramethoxysilane (TMOS)-based precursor mixture. Makoto Ogawa et al., Preparation of Self-Standing Transparent Films of Silica-Surfactant Mesostructured Materials and the Conversion to Porous Silica Films, Adv. Mater., 10, No. 14 (1998). The method described in Ogawa et al. includes dissolving water in a TMOS-based mixture. Some oligomerization of the silicate monomers begins before surfactants are added, but the intermediate resin is not isolated from the volatile byproducts before forming the films.
Other silica materials known in the art include powders templated with nonionic polyethylene oxides and amines, respectively, synthesized from TEOS mixtures (Thomas J. Pinnavaia et al., Unstable Mesostructured Silica Vesicles, Science Vol. 282 (Nov. 13, 1998); Louis Mercier, Direct Sythesis of Functionalized Silica by Non-Ionic Alkylpolyethyleneoxide Surfactant Assembly, Chem. Commun., pp. 1775-1776 (1998)) and gel bioencapsulants, notable for their high glycerol content and low conversion to silicate network, formed by mixing glycerol into a TEOS resin. Iqbal Gill et al., Encapsulation of Biologicals within Silicate, Siloxane, and Hybrid Sol-Gel Polymers. An Efficient and Generic Approach, J. Am. Chem. Soc. 120, pp. 8587-8598 (1998).
It is further known that the xe2x80x9cL3xe2x80x9d phase formed by a mixture of cetylpyridinium chloride (CPC), hexanol, and water can be used to template the growth of an ordered silicate solid from TMOS. The method provides good optical clarity and pore uniformity, but the solid products are extremely unstable to air, pulverizing spontaneously upon evaporation of volatiles. The curing process requires elevated temperatures (i.e. above room temperature) and long times, and gives products containing no more than 25% silica. K. M. McGrath et al, Formation of a Silicate L3 Phase with Continuously Adjustable Pore Sizes, Science Vol. 277 pp. 552-555 (1997). Additionally, a procedure for the synthesis of a pre-oligomerized resin from TMOS has been devised. Adachi and Sawai, Japanese Patent No. 07048454 A2 950221 Heisei.
It is advantageous to provide silicate materials with high silicon content that are more stable to annealing, drying and solvent exchange than analogs made from purely monomeric silicates.
It is further advantageous to provide silicate materials that have lower curing temperatures and shorter curing durations than known analogs.
It is also advantageous to provide silicate materials that can be formed into monoliths with thicknesses above 1 mm.
It is also advantageous to provide silicate materials with optimum mechanical strength and optical clarity.
It is further advantageous to provide silicate materials with lessened perturbation of the templating phase during resin cure.
The invention relates to a silicate material, comprising silicate domains and one or more substantially nonsilicate domains. The material is produced by mixing a templating mixture with a precured resin and preferably one or more resin precursors. The templating mixture preferably comprises one or more surfactants, one or more alcohols and water. A precured resin is formed by reacting one or more silicate resin precursors with water, preferably in the presence of a co-solvent and a catalyst. The precured resin is mixed with the templating mixture and preferably with an additional amount of one or more silicate precursors. Once the material has solidified, solvent may be exchanged within the substantially nonsilicate domains.
The invention also includes a process for producing a silicate material.
The invention further includes an optical article, a method for fabricating an optical article and a holographic medium.
The invention relates to a silicate material comprising silicate domains and one or more substantially nonsilicate domains, in particular to porous silicate materials, a method for fabricating a silicate material, and a holographic method utilizing a silicate material. (xe2x80x9cNonsilicate domainsxe2x80x9d as used herein shall include xe2x80x9csubstantially nonsilicate domainsxe2x80x9d)
The silicate materials of the invention are derived from pre-oligomerized resins, designed for maximum silicon content in the final product. The materials are particularly stable to annealing, drying, and solvent exchange compared to analogs made from purely monomeric silicates. Advantageously, the silicate material can be formed into monoliths with thicknesses greater than about 1 mm. Template and resin compositions and processing conditions can be varied to optimize the structural integrity and optical clarity of the silicate materials. An additional advantage of the invention is believed to be the lessened perturbation of the templating phase during resin cure because of the reduced amount of alcohol-leaving group expelled.
The term silicate, used herein, means a substantially crosslinked network generally derived from tetra-oxygenated silicon species, including , but not limited to, materials that are predominantly silicon oxide. Materials in which trioxygenated silicon is incorporated, such as those derived at least in part from organotrialkoxysilanes, are also considered silicates for the purposes of the invention. Incorporation of other element oxides into silicates, for example, Ge, B, Al, Ti and/or Zr oxides, by incorporation of their precursors into resin precursor mixtures is also within the scope of the invention.
As the volume percent of silicate in the material increases the structural strength increases. It is generally preferred for the silicate domain to comprise greater than about 15 volume percent. However, it is sometimes necessary to balance structural integrity with other desirable characteristics provided by the substantially nonsilicate domain. For example, for holographic applications, wherein the nonsilicate domain is a photosensitive medium, the photosensitive medium preferably comprises greater than 10 volume percent, more preferably greater than 20 volume percent, and most preferably greater than 25 volume percent of the material. Where a low dielectric constant (low-k) material is desired, the nonsilicate domain is generally a gas or vacuum, preferably comprising greater than 10 volume percent, more preferably greater than 20 volume percent, and most preferably greater than 25 volume percent of the material.
Materials have been produced with nonsilicate domains comprising the following: atmospheric gases, solvents, surfactants, monomers, and polymers. Domains comprising monomers may further comprise polymerization photoinitiators. It is also understood that the following components may be used in nonsilicate domains: oligomers, conductors, semiconductors, high dielectric constant materials, high or low refractive index materials, and precursors to the previously-listed materials. Furthermore, it is understood that a combination of any of the above second constituents may be utilized to form silicate materials within the scope of the invention.
The process by which the silicate materials may be formed can be divided into the following five general steps: 1) template formation; 2) initial resin formation; 3) final resin mixture formation; 4) shaping and curing; and 5) modification of the nonsilicate domain.
The templating mixture provides a means to tune the material morphology. The phase arrangement produced by the templating mixture generally depends on the quantity and types of mixture components. The templating mixture preferably comprises one or more surfactants, one or more alcohols and water. The surfactant(s) is preferably selected from the group comprising cetyltrimethylammonium bromide (CTAB), Brij 30(trademark) (the monododecyl ether of tetraethylene glycol) and cetylpyridinium chloride (CPC). Other surfactants with similar molecular functionality, i.e. a chain such as an alkyl or alkylaryl group of about 10-20 carbon atoms, having a nonpolar or hydrophobic end, and a polar or hydrophilic end comprising a group such as a quaternary ammonium, ethylene oxide oligomeric unit, sulfonate, sulfate, phosphate, or phosphonate group, are contemplated for use in the invention. Other molecular connectivity that produces a similar arrangement of polar and nonpolar functionalities are also contemplated for use in the invention. The surfactant can also be a mixture of surfactants.
The alcohol(s) is preferably a moderately polar alcohol which has about four to about ten carbon atoms and an OH group at or near the end of the chain. Examples of moderately polar alcohols include, but are not limited to, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, and alcohols of similar polarity such as a 2-hexanol or 2-methyl- 1-pentanol. The alcohol can also be a mixture of alcohols.
A catalyst, as discussed below, is also desirably incorporated into the templating mixture, to promote curing of the final resin mixture. The catalyst or other additives may also be chosen to favorably influence the micro structure of the templating mixture.
The initial resin formation step typically includes partially reacting a silicate resin precursor with water, in the presence of a cosolvent and a catalyst and preferably above ambient temperature. Resin formation generally takes place at temperatures of about 120xc2x0 C. but may take place at other temperatures, depending on the specific components used. During the reaction volatile components are substantially removed. The cosolvent may be selected from the group comprising methanol, ethanol, 1-propanol and isopropanol or a mixture thereof. It is also understood that this step may be performed without a cosolvent, and without a catalyst.
Advantageously, the use of a cosolvent assures homogeneous mixing of the resin precursor with the water and catalyst, preventing undesirable inhomogeneities in the reaction such as formation of gel particles. It is preferable that the cosolvent be volatile, so as not to become excessively incorporated in the resin or the final material. Where the cosolvent and the alcohol expelled by the hydrolyzing and condensing silicate precursors are not identical, it is advantageous for the cosolvent to promote the volatilization of the expelled alcohol, for example, by formation of an azeotrope.
The catalyst is typically a volatile acid and is generally selected from the group comprising HCL, BCl3, SiCl4, HNO3, CF3COOH, and HBr. The catalyst provides sufficient acidity for the hydrolysis and condensation of the silicate precursors to proceed at a convenient rate. The acidic function can be provided by a catalyst reagent directly, or by chemical transformation of a reagent, such transformation occurring, for example, through hydrolysis, thermal decomposition, or photoinduction. Preferably, the catalyst or catalyst byproducts do not deleteriously affect subsequent-desired activity of the material, e.g. for a holographic application, the catalyst should not promote the degradation of the recording medium, and for a low-k dielectric, should not lead to undesired dielectric breakdown. Catalysts selected from the group described above are also suitable for incorporation into the templating mixture or final resin mixture.
The resin precursor may be selected from the group comprising methyltrimethoxysilane, triethoxysilane, 1,4-bis (trimethoxysilylethyl) benzene (BSEB), tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) and mixtures comprising these silanes. Other small tetraalkoxy and trialkoxy silanes and silanes generally with three or four leaving groups, such as mixtures of chloro, acetoxy, and alkoxy groups, can be incorporated into the resin precursor. Dialkoxysilanes and monoalkoxysilanes may be incorporated, but not in quantities that unduly lower the crosslink density and thereby lead to softening of the final silicate material. Silanes with larger substituents, such as phenyl rings, may be incorporated, and such substituents may impart desirable functionality, such as a high refractive index. Again, these should not be introduced in quantities large enough to adversely affect the microstructure or mechanical strength of the final silicate material. Similar considerations apply to other element oxides that may be incorporated. The precursor compounds should be chosen so that their reactivity is suitable for forming a homogeneous resin, i.e. not so reactive as to preclude forming a homogeneous mixture with water and cosolvent, and in cases where a resin precursor mixture is used, one component of the mixture should not be so much more or less reactive than another that the less reactive precursor is not sufficiently incorporated into the resin.
Certain advantages are associated with particular resin precursors. For example, TMOS provides a high volume fraction of silicate domain, leading to high mechanical strength, because of the relatively small size of the alcohol (methanol) expelled when the resin derived therefrom is cured, compared to, for example, the ethanol expelled from TEOS-based resins. The use of exclusively tetra-oxygenated silicate precursors leads to silicate domains that are largely SiO2, which imparts a high modulus and low coefficient of thermal expansion to the final material. On the other hand, incorporation of organotrialkoxysilanes leads to silicate domains in which organosilyl groups are retained. These can impart increased toughness and a more hydrophobic silicate domain surface that would be desirable where the nonsilicate domains are to be filled with nonpolar materials. The use of organic groups bridging more than one trialkoxysilane as resin precursors provides the advantage associated with organosilanes without unduly sacrificing crosslink density.
The final resin mixture formation includes combining the precured resin from the initial resin formation step, with the templating mixture, and preferably with an additional amount of resin precursor to form the final resin mixture. The additional resin precursor can be the same as was utilized in the initial resin formation step. It is also understood that a different resin precursor may be used in the final resin mixture formation step.
Advantageously, it has been found that final resin mixture formation accomplished by including a substantial amount of precured resin produces more stable materials than if the precured resin had not been utilized. The materials are more stable to annealing, drying and solvent exchange. Additionally, thicker material may be formed by this process. Also, advantageously, less perturbation of the templating phase by expelled alcohol is achieved because less alcohol is expelled during curing than if no precured resin has been used.
Formation of the material shape has been accomplished by molding, but it will be understood by those skilled in the art that formation techniques may include casting, printing, extruding, injection molding, coating or the like. Materials may be further contoured after the initial shaping by methods such as carving, skiving, embossing or the like. The material may be fully or partially cured prior to shaping. Partially cured material may continue to cure during the shaping process. Curing has been performed at room temperature but, as will be appreciated by those skilled in the art, curing may be done at other temperatures or by exposure to other energy sources, for example microwave energy or ultra violet light.
Shaped and cured materials may be released from their containers or apparatus into a gaseous atmosphere or a liquid medium. The gaseous atmosphere can be air or constituents of air or other common gases or solvent vapors. Liquids can include, but are not limited to, solvents such as methanol, ethanol, isopropanol, butanol, combinations of these solvents, and mixtures of these solvents with water. Materials released into solvents have been found in some cases to be more stable to further manipulation such as heating and exchange of nonsilicate domain constituents than if released directly into air. Solvents are advantageously selected so that their interactions with silicate domain surfaces are closely matched to the interactions between the surfaces and the materials that already constitute the nonsilicate domains. In cases where it is desirable for the nonsilicate domain composition to be altered, especially by addition of a quantity of a solvent, it is advantageous that the solvent be miscible with materials that are to be removed from the nonsilicate domains. In such cases, the removed material is extracted by the solvent and most of the extract is advantageously transported to a solvent pool outside the silicate material.
The characteristics of the final material may be varied by varying the types and ratios of mixture components for templating and resin formation, and varying process conditions. Component variables that may be adjusted include, resin and surfactant molecular weight and functionality, water concentration, the nature of the alcohols, resin precursors, and catalysts, and the reactivity of resins and resin precursors. Process condition variables include, duration and method of mixing and degassing, temperature and duration of curing, and the degree to which volatiles are retained or released at different stages of cure.
Characteristics that can be adjusted by the above-mentioned variations include, but are not limited to, porosity, dielectric constant, structural stability, refractive index contrast and reactivity. Pores comprising a gas or vacuum tend to lower the dielectric constant of the material. If the pores are isolated from one another, dielectric breakdown is minimized. Advantageously, the dielectric constant is less than 3. The lower the dielectric constant, the lower the capacitive contribution to the delay time of a circuit employing the material as an insulator, and the less material needed to isolate electrical layers from one another. Thin dielectrics are particularly useful in the electronics industry where it is desirable to minimize device size, for example, as in ultra large scale integration.
Interconnected pores are desirable for holography applications where migration of molecules, such as monomers, within the material is necessary. Often existing materials for holography applications, such as Corning Glass"" Vycor(copyright), have rigid networks with small pores and have less ability to allow molecule movement than the silicate material of the present invention. Structural stability and reactivity can also be varied to optimize the material for a particular application.
Nonsilicate domains may be introduced into the material during or after resin formation and may be further adjusted after introduction into the material. Nonsilicate domains may be introduced by fluid exchange using techniques such as, vapor transport, solvent exchange, supercritical extraction, vacuum or drying.
As part of the process of adjusting the composition of the nonsilicate domains, the material may be immersed or heated in the presence of a fluid. Such fluids may be atmospheric gases, solvents, solvent vapors, and supercritical solvents. Examples of solvents that have been employed with the materials of the invention include, isopropanol, cyclohexanol, and toluene. Additionally, it will be understood by those skilled in the art that solvents may include, but are not limited to, alcohols, aromatic and alicyclic hydrocarbons, esters, ethers, and halogenated hydrocarbons. It is also possible for the solvent to comprise a mixture of these compounds. It is further possible for the solvent to comprise one or more solutes that need not necessarily be liquids in their pure forms. The solutes may comprise or be reagents that produce, oligomers, conductors, semiconductors, high dielectric constant materials, and/or high or low refractive index materials. Reactive compounds, may also be employed as solvents, for example, polymerizable monomers and photoinitiators, or precursors to materials with specific activities such as electrical conductivity or polarizability. As discussed above, where it is desirable to extract material from the nonsilicate domains, it is preferable that the material to be extracted be miscible, and more preferable that it be substantially soluble, in the solvent.
Supercritical solvents are an advantageous class of fluids. The use of these fluids in producing porous silicate materials is discussed in C. J. Brinker, xe2x80x9cSol-Gel-Sciencexe2x80x9d, Academic Press, New York (1990). Supercritical solvents facilitate material exchange in the nonsilicate domains with much less cracking and other deformation, because the capillary forces involved in the introduction and removal of supercritical fluids are much less than those associated with conventional solvents. Suitable solvents include, but are not limited to, inert compounds such as CO2 and hydroxylic solvents. Treatment with supercritical solvents results in materials with nonsilicate domains substantially free of condensed phase material, providing desirable properties such as low dielectric constant. Alternatively, the free volume in the nonsilicate domains may be exchanged for compounds with specific properties or activities, as discussed above.
It is also possible for material to be removed from the nonsilicate domains by vacuum action.
It is possible for solvent treatment to modify the silicate domains, especially when the process comprises heating. For example, the domains may become further crosslinked, and/or the functional groups on the surfaces may be changed. These changes may be advantageous, in that they can lead to enhancements in properties such as mechanical strength and wettability of nonsilicate domain material.
Initial experiments were performed to evaluate silicate materials formed without precuring a resin. Comparative Example 1 describes formation of such a silicate material.